Vertical thin film transistor

ABSTRACT

A semiconductor device includes a stack of layers stacked vertically and including a source layer, a drain layer and a channel layer between the source layer and the drain layer. A gate electrode is formed in a common plane with the channel layer and a gate dielectric is formed vertically between the gate electrode and the channel layer. A first contact contacts the stack of layers on a first side of the stack of layers, and a second contact formed on an opposite side vertically from the first contact.

BACKGROUND Technical Field

The present invention generally relates to field effect transistors(FETs), and more particularly to vertically disposed FETs with highresistance read out for cognitive device circuits.

Description of the Related Art

Cognitive device circuits can include neural networks or other machinelearning device structures. Typical transistor operation prefers highercurrent to reduce signal delay. However, higher current transistordevices may not be suitable for many cognitive device circuits, whichmay need lower current specifications for proper operation.

SUMMARY

In accordance with an embodiment of the present invention, asemiconductor device includes a stack of layers stacked vertically andincluding a source layer, a drain layer and a channel layer between thesource layer and the drain layer. A gate electrode is formed in a commonplane with the channel layer and a gate dielectric is formed verticallybetween the gate electrode and the channel layer. A first contactcontacts the stack of layers on a first side of the stack of layers, anda second contact is formed on an opposite side vertically from the firstcontact.

Another semiconductor device includes a substrate having front end ofthe line (FEOL) devices formed thereon and a cross bar grid includingfirst lines and second lines formed transversely to the first lines.Back end of the line (BEOL) vertical transistors are formed over aninterlevel dielectric layer, the vertical transistors each including astack of layers stacked vertically and including a source layer, a drainlayer and a channel layer between the source layer and the drain layer;a gate electrode formed in a common plane with the channel layer; a gatedielectric formed vertically between the gate electrode and the channellayer; a first contact connecting the source layer to a first line ofthe first lines; and a second contact formed on an opposite sidevertically from the first contact and connecting the drain layer to asecond line of the second lines.

A method for forming a semiconductor device includes forming a firstcontact through an interlevel dielectric layer (ILD); forming a stack oflayers on the ILD layer over the first contact, the stack of layersincluding a source layer, a drain layer and a channel layer between thesource layer and the drain layer; forming a spacer layer over the ILDlayer; conformally depositing a gate dielectric over the spacer layerand the stack of layers; forming a gate electrode in a common plane withthe channel layer over the gate dielectric on the spacer layer; andforming a second contact on an opposite side vertically from the firstcontact.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a schematic diagram showing a charge/discharge circuit alongwith a crossbar grid for a employing a low mobility and high resistancereset transistor in accordance with an embodiment of the presentinvention;

FIG. 2 is a cross-sectional view showing a back end of the line (BEOL)region having metal structures for forming connections to a thin filmtransistor to be formed in accordance with an embodiment of the presentinvention;

FIG. 3 is a cross-sectional view of the region of FIG. 2 showing adielectric barrier deposited in accordance with an embodiment of thepresent invention;

FIG. 4 is a cross-sectional view of the region of FIG. 3 showing thedielectric barrier opened up over a metal region in accordance with anembodiment of the present invention;

FIG. 5 is a cross-sectional view of the region of FIG. 4 showing a metalbarrier layer formed on the dielectric barrier and in the opening of thedielectric barrier and the formation of a stack of layers for forming asource, drain and channel layer for the TFT device to be formed inaccordance with an embodiment of the present invention;

FIG. 6 is a cross-sectional view of the region of FIG. 5 showing metalbarrier layer and the stack of layers for forming a source, drain andchannel layer for the TFT device patterned to size and shape the TFTdevice in accordance with an embodiment of the present invention;

FIG. 7 is a cross-sectional view of the region of FIG. 6 showing aspacer layer formed on the dielectric barrier in accordance with anembodiment of the present invention;

FIG. 8 is a cross-sectional view of the region of FIG. 7 showing a gatedielectric formed over the stack of layers and the spacer layer, and agate metal formed on the gate dielectric in accordance with anembodiment of the present invention;

FIG. 9 is a cross-sectional view of the region of FIG. 8 showing thegate metal patterned to form a gate conductor that is associated with aposition of the channel layer of the TFT device in accordance with anembodiment of the present invention;

FIG. 10 is a cross-sectional view of the region of FIG. 9 showing adielectric material formed on the gate conductor of the TFT device inaccordance with an embodiment of the present invention;

FIG. 11 is a cross-sectional view of the region of FIG. 10 showing aninterlevel dielectric layer formed in accordance with an embodiment ofthe present invention;

FIG. 12 is a cross-sectional view of the region of FIG. 11 showing theinterlevel dielectric layer patterned to form openings for metalstructures and contacts in accordance with an embodiment of the presentinvention;

FIG. 13 is a cross-sectional view of the region of FIG. 12 showingcontacts and metal lines formed in the interlevel dielectric layer inaccordance with an embodiment of the present invention;

FIG. 14 is a cross-sectional view showing the BEOL TFT device formed ina BEOL region and a front end of the line (FEOL) device formedtherebelow in accordance with an embodiment of the present invention;and

FIG. 15 is a block/flow diagram showing methods for forming a lowmobility, low current, high resistance vertical TFT in accordance withembodiments of the present invention.

DETAILED DESCRIPTION

Aspects of the present invention are directed to high resistance fieldeffect transistors (FETs). The high resistance FETs can be employed forread out operations for memory devices and are particularly useful incognitive devices, which can include neural networks and the like. Thehigh resistance FETs can include low mobility properties that result inlower current. These properties can be provided by materials and dopinglevels in the source, drain and channel layers of the TFT device.

In useful embodiments, the high resistance FETs can be included incrossbar circuits where the high resistance FETs are part of a circuitconnecting metal lines in rows and columns of a crossbar grid. In otherembodiments, the TFT device can be employed in display applications orthe like. While typical transistor operation prefers higher current dueto low delay performance, some applications, such as, cross barcognitive applications, need lower current or higher resistance toprovide satisfactory operation.

Embodiments of the present invention include a new vertical thin filmtransistor (TFT) device that can include low mobility and hence lowcurrent. The TFT can be fabricated at a substrate surface as a front endof the line device (FEOL) or as a back end of the line (BEOL) device orcombinations or variations thereof. The TFT built at the BEOL can beprovided closer to wiring and a BEOL capacitor, which are oftenfabricated later in the fabrication process. Such placement can haveadvantages in reducing delay and other electronic benefits as well asefficient use of available chip area. The TFT is preferably verticallydisposed to provide ease of manufacture as well as chip area efficiency.

In useful embodiments, methods for forming vertical TFTs in accordancewith aspects of the present invention are provided. While the verticalTFT can be formed at or near a substrate level (e.g., FEOL), the methodswill describe details for formation of the vertical TFT at the BEOL fora semiconductor device or chip.

It is to be understood that aspects of the present invention will bedescribed in terms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps can be varied within the scope of aspects of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”,as well as other variations thereof, means that a particular feature,structure, characteristic, and so forth described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrase “in one embodiment” or “in an embodiment”, as well anyother variations, appearing in various places throughout thespecification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including.” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a schematic diagram shows acircuit 10 having a readout field effect transistor (FET) 20. Thereadout FET 20 is employed between nodes A and B to read out chargestored in a capacitor 22. The capacitor 22 is charged or discharged inaccordance with inputs V₁ and V₂, which control the charging anddischarging of the capacitor 22 using a charge transistor 26 and adischarge transistor 24. The charge transistor 26 and dischargetransistor 24 can include a p-type FET (PFET) and an N-type FET (NFETrespectively; however, these devices and the readout transistor 20(e.g., an NFET) can have reversed polarities as needed.

It should be understood that the circuit 10 can include additional ordifferent components and may perform similar or completely differenttasks (e.g., a pixel transistor for a display device, etc.), as needed.In one useful embodiment, the readout FET 20 is included in a crossbararray 30. The crossbar array 30 includes a grid of transverse metallines 12, 14. One set (e.g., lines 12) of the metal lines can includebitlines; while the other set, e.g., lines 14, can include wordlines, orvice versa. Sources and drains (nodes A and B) of the readout FET 20 arecoupled to the lines 12, 14.

In one embodiment, the crossbar array 30 forms a neural network or othercognitive device that simulates neurons. A neural network stores pathinformation that simulates learning in devices. The properties of thereadout FET 20 can be altered to provide responsiveness that simulateslearning. In one embodiment, high resistance or low current can beemployed to serve the objective of the array 30 as a cognitive device.Other uses and methods can also be employed.

Referring to FIG. 2, a cross-sectional view of a semiconductor device100 is shown in a partially fabricated state. In this example, thedevice 100 has undergone previous processing to form a dielectric layer102 over front end of the line (FEOL) structures. FEOL structures caninclude, e.g., transistors formed in a substrate (with diffusionregions), contacts, metal lines, dielectric layers, etc. While thepresent embodiments can be employed to form devices at the FEOL, thepresent example will show and describe the formation of a vertical thinfilm transistor (TFT) at or close to the back end of the line (BEOL) ina fabrication cycle for the device 100. The dielectric layer 102 caninclude an interlevel dielectric (ILD) layer.

The device includes metal structures 104, 106, 108, 110 and 112 formedin a dielectric layer 102. The dielectric layer 102 can includedielectric materials, such as, e.g., low-k dielectrics, oxides,nitrides, and oxynitrides of silicon or other suitable dielectricmaterials. The metal structures 104, 106, 108, 110 and 112 can includeany suitable conductive material, such as doped polycrystalline, metal(e.g., tungsten, titanium, tantalum, ruthenium, zirconium, cobalt,copper, aluminum, lead, platinum, tin, silver, gold, etc.), a conductingmetallic compound material (e.g., tantalum nitride, titanium nitride,tungsten silicide, tungsten nitride, ruthenium oxide, cobalt silicide,nickel silicide), carbon nanotube, conductive carbon, graphene, or anysuitable combination of these materials in one or more layers.

The metal structures 104, 106, 108, 110 and 112 can include contacts,metal lines or other electrically conductive components. The metalstructures 104, 106, 108, 110 and 112 can be arranged in any suitableconfiguration. A planarization process or the like may be employed toplanarize a top surface of the device 100 and expose structures 106 and112. The planarization process can include an etch process or a chemicalmechanical polish (CMP).

Referring to FIG. 3, a dielectric barrier 114 is formed over the surfaceof the device 100. The dielectric barrier 114 can include a nitride, anN-block or other suitable dielectric material. In one embodiment, thedielectric barrier includes a thickness of between about 10 nm to about20 nm, although other thicknesses may be employed. The dielectricbarrier 114 can be formed by chemical vapor deposition (CVD), physicalvapor deposition (PVD) or any other suitable deposition process.

Referring to FIG. 4, the dielectric barrier 114 is patterned to form anopening 116 over the metal structure 112. The patterning process caninclude photolithography or any other suitable patterning process. Thepatterning process leaves the metal structure 112 exposed so that aconductive material formed in a next step can make electrical contactwith metal structure 112.

Referring to FIG. 5, a stack 126 of layers are formed on the device 100.The stack 126 of layers includes the formation of source, channel anddrain regions for a vertical TFT device. A barrier metal layer 118 isdeposited to fill the opening 116, make contact with the metal structure112 and cover the dielectric layer 114. The barrier metal layer 118 caninclude a metal compound to function as a diffusion barrier, such as aconducting metallic compound material or metal (e.g., tantalum nitride,titanium nitride, tantalum, tungsten nitride, TaSiN, combinations ofthese and other materials). The barrier metal layer 118 can be depositedusing a chemical vapor deposition process (CVD), physical vapordeposition (PVD) or other suitable deposition process.

The stack 126 further includes layers 120, 122 and 124. These layers canbe formed together or separately. In one embodiment, the layers 120, 122and 124 are formed from polysilicon. Following deposition of thepolysilicon layer(s), the deposited polysilicon can be doped with anappropriate dopant, or alternatively, an in-situ doping depositionprocess is employed in forming the polysilicon layer. In anotherembodiment, the layers 120, 122 and 124 are formed separately and dopedin-situ or after formation, as needed. The deposition of the layers 120,122 and 124 can include a CVD process, PVD process (e.g., evaporation)or any other suitable process. Using polysilicon or amorphous silicon,permits ease of manufacture of the TFT to be formed. Althoughmonocrystalline materials are contemplated for layers 120, 122, 124,these devices can be more easily formed at FEOL locations.

The doping amount and conductivity type of the layers 120, 122 and 124of the stack 126 will depend on the type of device being formed. In oneexample, the device to be formed can include an NFET device, and thelayer 120 can be doped with n dopants with an n+ concentration, thelayer 122 can remain undoped, and the layer 124 can be doped with ndopants with an n+ concentration. In another example, the device to beformed can include a PFET device and the layer 120 can be doped with pdopants with a p+ concentration, the layer 122 can remain undoped andthe layer 124 can be doped with p dopants with a p+ concentration.

While sources and drains can be juxtaposed, the embodiment described caninclude a source layer 120 and a drain layer 124. In the example shown,the layer 122 forms a channel layer 122 for a vertical TFT device to beformed. In one embodiment, the layers 120, 122 and 124 have equalthicknesses although different thicknesses can be employed. Thethicknesses of layers 120, 122 and 124 can include 50 nm-200 nm toprovide a vertical TFT structure. Metal structures 108, 110 and 112provide a connection to the source layer 120 through the barrier metallayer 118 (if employed).

In accordance with aspects of the present invention, the size (e.g.,thickness), shape and doping levels of the layers 120, 122 and 124 canbe controlled to tune or otherwise provide resistance control of thetransistor device to be formed. In this way, the higher resistancedevices can be created that are particularly suitable for cognitivedevices, e.g., with crossbar grid structures. In addition, the use ofpolycrystalline materials such as, e.g., polysilicon, adds resistanceand therefore reduces current in the transistor to be formed.

Referring to FIG. 6, a patterning process is performed to pattern andshape the stack 126 to form a transistor shape 128. The transistor shape128 can include any useful shape when viewed from a top of the device100 down toward layer 124. The shape can include a round shape, an ovalshape, a polygonal shape (e.g., square, rectangle triangle, octagon,etc.). The patterning process can include a lithographical patterningprocess or any other patterning process.

In one example, a photoresist can be deposited over the stack andexposed to light to form a resist mask. Then, an anisotropic etch, suchas, e.g., a reactive ion etch (RIE) process can be performed to etch thestack 126 down to the dielectric layer 114. A single etch process can beperformed for all layers of the stack 126 or multiple etch processes canbe employed depending on the materials and the structure.

Referring to FIG. 7, a spacer layer 130 is deposited over the device100. The spacer layer 130 can include any dielectric material including,e.g., silicon oxide, silicon nitride, silicate glasses or any othersuitable dielectric. The spacer layer 130 initially covers that layer124. A planarization process, such as a CMP, can be employed to removethe dielectric material of the spacer layer 130 down to the layer 124.Then, a selective recess etch can be employed to recess the dielectricmaterial to a height 132 relative to the shaped transistor 128. Therecess etch can include a wet or dry etch process selective to thematerials of the shaped transistor 128, e.g., layers 120, 122, 124. Theheight 132 defines a position for a gate conductor to be formed. Thegate conductor should be relatively aligned with the channel layer 122and will sit on top of the spacer layer 130.

Referring to FIG. 8, a dielectric layer 134 is conformally formed overthe device 100. The dielectric layer 134 forms a gate dielectric betweenthe channel layer 122 and a gate conductor layer 136 formed over thedielectric layer 134. The dielectric layer 134 can be formed using CVD,PVD or other suitable deposition processes.

The dielectric layer 134 can include an oxide, a nitride or othersuitable dielectric material. In one embodiment, the dielectric layer134 includes a “high-k” dielectric material featuring a dielectricconstant (k) higher than the dielectric constant of SiO₂. High-kdielectric materials can include, but are not limited to, hafniumoxides, hafnium silicates, titanium oxides, barium-strontium-titantates(BSTs) and lead-zirconate-titanates (PZTs).

The gate conductor layer 136 includes a conductive material including,but not limited to metals, metal alloys, metal nitrides and metalsilicides, as well as laminates thereof and composites thereof. In oneembodiment, the gate conductor layer 136 may be any metal including, butnot limited to W, Ni, Ti, Mo, Ta, Cu, Pt, Ag, Au, Ru, Ir, Rh, and Re,and alloys that include at least one of the aforementioned conductiveelemental metals. The gate conductor layer 136 may also include dopedpolysilicon and/or polysilicon-germanium alloy materials (i.e., having adopant concentration from 1×10¹⁸ dopant atoms per cubic centimeter to1×10²² dopant atoms per cubic centimeter) and polycide materials (dopedpolysilicon/metal silicide stack materials).

In useful embodiments, the gate conductor layer 136 includes a metaldeposited using CVD, sputtering or other suitable deposition process.Examples of metals that may be employed for the at least one gateconductor layer may include, but are not limited to, W, Ti, Ta, Cu, Pt,Ag, Au, Al, TiN, WN, TaN, TiAlN, TaAlN, and alloys thereof.

The gate conductor layer 136 is positioned adjacently (e.g., coplanar)to the layer 122, which acts as a channel layer between source 120 anddrain 124. The gate conductor layer 136 is deposited, planarized (e.g.,CMP) and recessed (e.g., by a wet or dry etch) to control the thicknessand the top height of the gate conductor layer 136. Further, the gateconductor layer 136 is located at the appropriate location of the stack126 (e.g., centered on the channel layer although an offset can beprovided) and surrounds the channel layer 122 partially or completely onall sides of the transistor shape 128.

Referring to FIG. 9, the gate conductor layer 136 is patterned to form apatterned gate conductor 138. The gate conductor 138 can be patterned tocontrol the size and shape of the gate conductor 138. The gate conductor138 can fully or partially surround the transistor shape 128. Thepatterning process can include a lithographical patterning process orany other patterning process.

In one example, a photoresist can be deposited over the gate conductorlayer 136 and exposed to radiation (e.g., ultraviolet (UV) light) toform a resist mask. Then, an anisotropic etch, such as, e.g., a reactiveion etch (RIE) process can be performed to etch the gate conductor layer136 down to the gate dielectric layer 134.

Referring to FIG. 10, the gate conductor 138 needs to be electricallyisolated. A dielectric cap layer 140 is deposited over the device 100,e.g., by CVD, a spin on process or other suitable process. The cap layer140 can include an oxide, a nitride, or any other suitable dielectricmaterial. In one embodiment, the cap layer is deposited and thensubjected to a planarization process, e.g., a CMP process or etch toreduce the dielectric material to a level of the gate dielectric 134 onlayer 124. In one embodiment, the dielectric cap layer 140 can be lefthaving a thickness sufficient for forming other metal structures (e.g.,also form an interlevel dielectric (ILD) layer). Alternately, a seconddielectric layer (ILD 142, FIG. 11) can be formed to build up thethickness of the dielectric material for forming metal structures aswill be described.

Referring to FIG. 11, an interlevel dielectric layer (ILD) 142 is formedon the device 100. The ILD 142 can be formed concurrently with cap layer140 or may be formed in a separate process. In one embodiment, the ILD142 can include an oxide, a nitride, or any other suitable dielectricmaterial. In one embodiment, the ILD 142 include a same material as thecap layer 140. In other embodiments, different materials can be used forthe cap layer 140 and the ILD 142. The ILD 142 is deposited and then canbe subjected to a planarization process, e.g., a CMP process or etch toreduce the dielectric material to a desired level.

Referring to FIG. 12, the ILD 142 is patterned to form openings 144, 146and 148 therethrough. The openings 144, 146 and 148 can be formed by oneor more lithographic patterning and etch sequences. One etch process canform contact holes 150 with respect to a resist mask (not shown) andanother etch to form metal line trenches 152 with respect to anotherresist mask (not shown).

Opening 144 exposes the layer 124 to provide a connection path to thedrain of the transistor. Opening 146 exposes the gate conductor 138 toprovide a gate contact path. Opening 148 exposes the metal structure 106to provide a connection path thereto for other connections.

Referring to FIG. 13, a metal or conductive layer is deposited over thedevice 100 to fill openings 144, 146 and 148. A planarization process,such as CMP or an etch can be performed to remove the excess metal andform contacts 162, 166 and 170 and metal lines 164, 168 and 172.

The contacts 162, 166 and 170 and metal lines 164, 168 and 172 mayinclude any suitable conductive material, such as polycrystalline oramorphous silicon, germanium, silicon germanium, a metal (e.g.,tungsten, titanium, tantalum, ruthenium, zirconium, cobalt, copper,aluminum, lead, platinum, tin, silver, gold), a conducting metalliccompound material (e.g., tantalum nitride, titanium nitride, tungstensilicide, tungsten nitride, ruthenium oxide, cobalt silicide, nickelsilicide), carbon nanotube, conductive carbon, graphene, or any suitablecombination of these materials. The conductive material may furthercomprise dopants that are incorporated during or after deposition. Thecontacts 162, 166 and 170 and metal lines 164, 168 and 172 can be formedover a barrier material or liner (such as, TaN, TiN, etc. (not shown)).

A transistor 160 can be formed between metal lines 164 and 108 withinupper layer of the device 100. The transistor 160 can include atransistor 20 (FIG. 1) connected in a crossbar grid or other metalstructure configuration. In one example, the metal line 164 can includea metal line 14 (FIG. 1) and the metal line 108 can include a metal line12 (FIG. 1). As depicted in FIG. 13, the metal line 164 connects to A ofthe transistor 160 and the metal line 108 connects to B of thetransistor 160. Metal line 108 and metal line 164 can be transverselydisposed in a crossbar arrangement.

It should be understood that the transistor 160 and the metal lines 108,164 can be formed or arranged differently. For example, the metal lines108 and 164 can be formed in a grid below the transistor 160 or abovethe transistor 160. The transistor 160 could also be formed in a sameplane as one or more or the metal lines 108, 164. The metal lines andstructures can be configured to connect with the source and drain sidesof the transistor 160, as needed, using contacts and metal lines.

It should be further understood that the transistor 160 is verticallydisposed and includes horizontally stacked layers 120, 122, 124. Thehorizontally stacked layers 120, 122, 124 include a channel layer 122that is enabled using a gate electrode (138) that surrounds or at leastpartially surrounds the channel layer 122. In the illustrativeembodiment shown, the transistor 160 is formed within the BEOL portionof the device 100. However, transistors can be present at any ordifferent levels within the device.

Referring to FIG. 14, a schematic cross-sectional view of a cognitivedevice 200 shows a transistor 160 formed at the BEOL in accordance withembodiments of the present invention. The device 200 also shows a FEOLtransistor 210. The FEOL transistor 210 can include a verticaltransistor formed in a manner similar to that of transistor 160, butemploying monocrystalline or polycrystalline source, drain and channellayer materials.

In the embodiment shown, transistor 210 can include a planar transistor,a thin film transistor, a vertical transistor, a vertical TFT or anyother suitable transistor type. The transistor 210 can be formed at ornear a substrate 202 and be connected to other devices using contacts212 and/or metal lines 216, 214 formed in one or more ILDs 204. Thetransistor 160 is formed as described at the BEOL and connects to metallines (e.g., in a crossbar grid (not shown) using contacts 218 and 220.

Processing can continue with the formation of additional BEOL structuresincluding but not limited to ILDs, contacts, metal lines,metal-insulator-metal (MIM) capacitors, etc.

Referring to FIG. 15, methods for forming semiconductor devices areillustratively shown and described. In some alternative implementations,the functions noted in the blocks may occur out of the order noted inthe figures. For example, two blocks shown in succession may, in fact,be executed substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustration, and combinations of blocks in the blockdiagrams and/or flowchart illustration, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts or carry out combinations of special purpose hardware and computerinstructions.

In block 302, a first contact is formed through an interlevel dielectriclayer (ILD) (e.g., a prior or first ILD). This ILD can be in a FEOLregion, between FEOL structures and BEOL structures or in the BEOLregion. The first contact can be connected to FEOL structures or anyother suitable structures, e.g., lines of crossbar grid disposed in afirst direction. In block 304, in one embodiment, a dielectric barriercan be formed and patterned to form openings down to the first contact.In block 306, a metal barrier can be formed on the dielectric barrierand in the opening(s) in the dielectric barrier.

In block 308, a stack of layers is formed on the ILD layer (on thedielectric barrier or the metal barrier, if present) and over the firstcontact. The stack of layers includes a source layer, a drain layer anda channel layer disposed between the source layer and the drain layer.The source and drain layers can be similarly doped to form an NFET orPFET. In one embodiment, the channel layer is undoped. The stack oflayers are sized and dimensioned to form a vertical TFT. The materialsemployed, the doping levels and the shapes of the device can all beemployed to tune the mobility, the resistance and the current throughthe device. This control and the ability to employ a low mobility highresistance structure is useful in cognitive device applications. Thestack of layers can be deposited using a same base material for eachlayer of the stack wherein the source layer and the drain layer includea same doping conductivity and the channel layer is undoped. In otherembodiments, different materials can be employed for different layers.In one embodiment, a polycrystalline material is employed for the stackof layers and a same base material, such as, e.g., polysilicon isemployed for the source, drain and channel layers.

In block 310, the stack layers, which may or may not include the metalbarrier layer, are patterned to form a shape of the transistor to beformed. Lithographic or other patterning techniques can be employed.

In block 312, a spacer layer is formed over the first ILD layer (ordielectric barrier, if present) to provide a platform for a gate metalto be formed to position the gate metal even with the channel layer.

In block 314, a gate dielectric is conformally deposited over the spacerlayer and the stack of layers. In block 316, a gate electrode is formedin a common plane with the channel layer over the gate dielectric on thespacer layer. This can include a deposition process and a patterningprocess. The gate electrode can surround or partially surround thechannel layer with the gate dielectric disposed between gate electrodeand the channel layer. The gate electrode at least partially surroundsthe channel layer at a periphery of the channel layer.

In block 318, a dielectric cap layer and/or a second ILD are formed overthe gate electrode and the stack of layers. In block 320, a secondcontact is formed on an opposite side (vertical offset) from the firstcontact. The first contact and the second contact connect to thevertical TFT formed by the stack of layers. In block 322, processing cancontinue to complete the device. The vertical TFT may be formed ondielectric layers at a back end of the line region of the semiconductordevice. The vertical TFT can have its source layer connected to a firstmetal line with the first contact, and the drain layer can be connectedto a second metal line (which is transversely disposed to the firstmetal line) in a crossbar grid structure using the second contact.

Having described preferred embodiments for semiconductor devices andmethods for forming semiconductor devices (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A semiconductor device, comprising: a stack of layers stacked vertically and including a source layer, a drain layer and a channel layer between the source layer and the drain layer, the drain layer formed from a polycrystalline material; a gate electrode formed in a common plane with the channel layer; a gate dielectric formed vertically between the gate electrode and the channel layer; a first contact contacting the stack of layers on a first side of the stack of layers; a second contact formed on an opposite side vertically from the first contact; and a barrier metal layer interfacing between the polycrystalline material of the drain layer and the second contact.
 2. The semiconductor device as recited in claim 1, wherein the stack of layers includes a same base material.
 3. The semiconductor device as recited in claim 1, wherein the source layer and the drain layer include a same doping conductivity and the channel layer is undoped.
 4. The semiconductor device as recited in claim 1, wherein the gate electrode completely surrounds the channel layer at a periphery of the channel layer.
 5. The semiconductor device as recited in claim 1, wherein the gate electrode partially surrounds the channel layer at a periphery of the channel layer.
 6. The semiconductor device as recited in claim 1, wherein the semiconductor device is formed on dielectric layers at a back end of the line region of the semiconductor device.
 7. The semiconductor device as recited in claim 1, wherein the source layer connects to a first metal line and the drain layer connects to a second metal line transversely disposed to the first metal line in a crossbar grid structure.
 8. The semiconductor device as recited in claim 1, wherein the source layer, the drain layer and the channel layer include a polycrystalline material selected to increase device resistance and reduce device current.
 9. The semiconductor device as recited in claim 8, wherein the polycrystalline material includes polysilicon.
 10. The semiconductor device as recited in claim 1, wherein the source layer, the drain layer and the channel layer each include a same thickness.
 11. The semiconductor device as recited in claim 10, wherein the same thickness is between about 50 nm to about 200 nm.
 12. A semiconductor device, comprising: a substrate having front end of the line (FEOL) devices formed thereon; a cross bar grid including first lines and second lines formed transversely to the first lines; back end of the line (BEOL) vertical transistors formed over an interlevel dielectric layer, the vertical transistors each including: a stack of layers stacked vertically and including a source layer, a drain layer and a channel layer between the source layer and the drain layer, the drain layer formed from a polycrystalline material; a gate electrode formed in a common plane with the channel layer; a gate dielectric formed vertically between the gate electrode and the channel layer; a first contact connecting the source layer to a first line of the first lines; a second contact formed on an opposite side vertically from the first contact and connecting the drain layer to a second line of the second lines; and a barrier metal layer interfacing between the polycrystalline material of the drain layer and the second contact.
 13. The semiconductor device as recited in claim 12, wherein the stack of layers includes a same base material.
 14. The semiconductor device as recited in claim 12, wherein the source layer and the drain layer include a same doping conductivity and the channel layer is undoped.
 15. The semiconductor device as recited in claim 12, wherein the gate electrode completely surrounds the channel layer at a periphery of the channel layer.
 16. The semiconductor device as recited in claim 12, wherein the gate electrode partially surrounds the channel layer at a periphery of the channel layer.
 17. The semiconductor device as recited in claim 12, wherein the source layer, the drain layer and the channel layer include a polycrystalline material selected to increase device resistance and reduce device current.
 18. The semiconductor device as recited in claim 17, wherein the polycrystalline material includes polysilicon.
 19. The semiconductor device as recited in claim 12, wherein the source layer, the drain layer and the channel layer each include a same thickness.
 20. The semiconductor device as recited in claim 19, wherein the same thickness is between about 50 nm to about 200 nm.
 21. A method for forming a semiconductor device, comprising: forming a first contact through an interlevel dielectric layer (ILD); forming a stack of layers on the ILD layer and over the first contact, the stack of layers including a source layer, a drain layer and a channel layer between the source layer and the drain layer; forming a spacer layer over the ILD layer; conformally depositing a gate dielectric over the spacer layer and the stack of layers; forming a gate electrode in a common plane with the channel layer over the gate dielectric on the spacer layer; and forming a second contact on an opposite side vertically from the first contact.
 22. The method as recited in claim 21, wherein forming the stack of layers includes depositing a same base material for each layer of the stack wherein the source layer and the drain layer include a same doping conductivity and the channel layer is undoped.
 23. The method as recited in claim 21, wherein the gate electrode at least partially surrounds the channel layer at a periphery of the channel layer.
 24. The method as recited in claim 21, wherein the semiconductor device is formed on dielectric layers at a back end of the line region of the semiconductor device.
 25. The method as recited in claim 21, further comprising connecting the source layer to a first metal line with the first contact and connecting the drain layer to a second metal line transversely disposed to the first metal line in a crossbar grid structure using the second contact. 